This invention refers to a rolling stand for plane products, such as strip or similar, and an associated method to control the planarity of said strip. The stand is advantageously of the six-high type, with a pair of working rolls (WR) bevelled at least at one end and associated with both negative and positive bending mechanisms and axial displacement, or shifting, mechanisms, a pair of back-up rolls (BUR), and a pair of intermediate rolls (IR) associated with crossing mechanisms.
The planarity control method provides that the quadratic components, the fourth order components and the edge-drop of the profile of the rolled strip are controlled in a coordinated manner. To be more exact, the quadratic and fourth order components can also be controlled with a high dynamic performance.
The state of the art includes six-high rolling stands for plane products, comprising a pair of working rolls, a pair of back-up rolls and a pair of intermediate rolls, wherein, to control the planarity of the rolled product both the working rolls and the intermediate rolls are associated with both negative and positive bending systems, and wherein the intermediate rolls are also associated with a system of long axial translation (macro shifting).
These rolling stands, however, have the disadvantage that they cannot completely and efficiently compensate edge-drop, and require a particularly long axial translation of the intermediate rolls.
Another disadvantage of such rolling stands, which provide for the shifting of the intermediate rolls, is that the speed at which the shifting is performed is extremely slow compared with the rolling speed, that is to say, about {fraction (1/1000)} of the latter. Therefore, if the setting of the stand is not correct, since the inlet profile of the product being rolled is different from the aspected profile, or the rolling force is different from the initial expected one, there is a delay in the re-setting of the stand, for example because of the speed of shifting, with a resulting loss of planarity for a length of strip which is equal to the time taken to reset the stand multiplied by the rolling speed. To at least partly solve this problem of compensating the edges, there have already been proposals for rolling stands with a system of axial translation of the working rolls in the same direction as the intermediate rolls and wherein the working rolls are equipped with appropriate bevels or hollows at the ends.
Moreover, the state of the art also includes a rolling stand wherein the intermediate rolls (IR) are associated with crossing means suitable to reduce the so-called xe2x80x9cstrip walkingxe2x80x9d.
The present Applicant has devised, designed and perfected a rolling stand for plane products and a method to control the planarity of said products according to the invention to overcome the shortcomings described above and to perfect the rolling stands known in the state of the art.
The rolling stand for plane products and the method to control the planarity of said products according to the invention is set forth and characterised in the main claims, while the dependent claims describe other innovative features of the invention.
One purpose of the invention is to achieve a rolling stand for plane products, such as strip or similar, which will make possible to control and adjust, autonomously and independently, also during rolling, the x2 component, the x4 component and also components of a higher order, which consequently makes it possible to control the edge-drop of the rolled product, that is to say, components up to x10.
In accordance with this purpose, the rolling stand for plane products according to the invention comprises a pair of working rolls, a corresponding pair of back-up rolls and at least an intermediate roll located between one of the working rolls and a corresponding back-up roll, shifting means and bending means associated with at least one of the working rolls to translate it axially and respectively bend it, and crossing means associated with the intermediate roll to arrange it with its longitudinal axis inclined, that is, rotated, with respect to the longitudinal axes of the working rolls and the back-up rolls.
Before describing the invention in detail, it is appropriate to make the following premises.
The ability to control the profile of the strip being rolled is generally shown in the plane x2, x4 (FIG. 5), where x2 and x4 are the second and fourth order components of the function y(x)=a0+a1x+a2x2+a3x3+. . . +a10x10, which represents the thickness of the strip (FIG. 6).
If the thickness is symmetrical, as it should be, the odd components should not be present. At most, we might find the component a1x which indicates the presence of strip with a wedge defect, that is, a profile which is on average trapezoid with edges of a different thickness, as shown in FIG. 7.
The more efficient a stand is at controlling the shape, the wider is the zone x2, x4 which can be controlled; FIG. 8 shows two areas, the most extensive of which refers to a system with a higher control capacity than the more inward area.
If a stand has high dynamic performance in controlling the shape of the strip, this means that it is possible to pass quickly from a point A (FIG. 9) to a point B in the plane x2, x4. Then, together with an area of xe2x80x9cstaticxe2x80x9d or preset control, an area of xe2x80x9cdynamicxe2x80x9d control is also shown, clearly included in the area of static control which moves inside the area of global control (FIG. 10) according to the initial static functioning point xe2x80x9c0xe2x80x9d.
Since every actuator suitable to control the movements of the working rolls and intermediate rolls, in every operating condition (that is, roll diameters, strip width, inlet profile, rolling force, etc.) has its own xe2x80x9cline of actionxe2x80x9d, to pass with complete freedom from a point A to a point B, it is generally necessary to have two actuators AT1 and AT2 which move in their own directions d1 and respectively d2 (FIG. 11). Therefore, in the field of dynamic control, to have the possibility to pass from A to B without constraints on position B, the two necessary actuators must also have lines of action which are not parallel.
This having been said, FIG. 12 shows the control of the crossing of an intermediate roll (IR) according to the invention, wherein it can be noticed how the influence of x2 has limited collateral effects on x4, since the ratio between x2 and x4 is about {fraction (1/10)}. Therefore, by acting on IR crossing we have very limited effects on the x4 component.
From the detail shown in FIG. 13, in which the two working rolls (WR) are shown, it can be seen how WR shifting prevalently influences the edges of the strip, if the working roll is appropriately bevelled.
WR shifting influences both x2 and x4 but in a very limited way compared with WR bending and IR crossing. WR shifting is practically defined by the width of the strip, with very small adjustments according to the actual edge-drop on the strip at outlet. The ratio between x2 and x4 is about 1.
As can be seen in FIG. 14, WR bending influences both x2 and x4. The ratio x4/x2 depends on the choice of the diameters of the rolls of the stand and on the width of the strip (rolling force, etc.), and is in any case near 1.
The influence of IR crossing and WR bending on the edges of the strip is very limited, and therefore when IR crossing and WR bending is varied, it is not necessary to modify the set of WR shifting.
Therefore, the rolling stand according to the invention is equipped with means which allow IR crossing, WR shifting and WR bending.
To be more exact, WR shifting is used to pre-set the working rolls according to the edge-drop.
The stand according to the invention provides to actuate a shifting action on the working rolls in order to allow an adequate control of the shape of the strip edges for all the strip width values provided for a given rolling stand.
The working rolls are therefore associated with translation means able to displace them axially one with respect to the other, with a travel which may even reach several tens of centimetres, so as to be able to process plane rolled products with very variable widths.
The value of the shifting travel xe2x80x9cSxe2x80x9d to be achieved can be defined by the following formula: S=(Lmaxxe2x88x92Lmin)/2+EC, where Lmaxxe2x88x92Lmin are respectively the values of maximum and minimum width of the strip to be worked, and EC represents the extra travel which has to be provided to allow the action of the bevels in correspondence with the edges.
In the case of a six-high stand for cold rolling, the values of maximum and minimum width are respectively of about 1,200 mm and 600 mm, therefore the shifting value xe2x80x9cSxe2x80x9d is at least 300÷350 mm, taking into account a certain value of extra travel.
In the case of a six-high stand for hot rolling, the values are respectively about 2,600 mm and 1,300 mm, therefore the shifting value xe2x80x9cSxe2x80x9d is at least 650÷700 mm.
This constitutes a xe2x80x9cstaticxe2x80x9d actuator which does not influence the field of control x2, x4 since it is constrained only to the desired edge-drop correction.
IR crossing is used to pre-set the IR to obtain a desired x2 component. IR crossing is obtained by means of a preset actuator which is however used in rolling too, to change the x2 component and allow, together with WR bending, a total dynamic control.
IR shifting, as in conventional stands, practically has an influence only on x2 (the x4/x2 ratio is equal to about {fraction (1/15)}), and has an x2 variation action reduced by about 3-4 times compared with those of IR crossing. The comparison is between IR shifting, with a travel for example of about 300 mm and IR crossing with a rotation of up to about 1.5xc2x0. Therefore IR crossing is much more efficient.
Moreover, IR shifting, where it is included, is variable in rolling, with shifting speeds of {fraction (1/1000)} of rolling speeds to prevent damage to the surfaces of the rolls. With a rolling speed of 20 m/sec we have a shifting speed of 20 mm/sec. Therefore, it would take 10 secs to carry out the whole control travel.
The IR crossing speed is higher, at about 0.1xc2x0/sec. Consequently, to have the same x2 variation corresponding to the whole shifting travel (in the embodiment which includes IR shifting), it is enough to vary the crossing angle by 0.2-0.6xc2x0 according to the starting point (FIG. 15). Moreover, crossing is quicker: 0.2-0.6xc2x0 are varied in 2-6 secs, whereas with IR shifting it needs at least 10 secs to carry out the whole travel and obtain the same effects on the strip.
Thanks to the high control capacity of IR crossing, which as we have seen is on average about three times more than that of IR shifting in conventional stands, it is possible to use IR crossing also in a five-high stand, keeping high the capacity to control the profile of the strip.
According to a preferential embodiment of the invention, a pair of intermediate rolls is located between the pair of working rolls and the pair of back-up rolls, therefore the rolling stand is the six-high type.
According to a simplified variant, only one intermediate roll is arranged in the upper section between a corresponding working roll and a corresponding back-up roll, therefore the stand is of the five-high type.
According to one characteristic of the invention, the bending of each working roll can be both positive and negative.
According to another characteristic of the invention, the working rolls are provided, at least at one end, with bevels appropriately configured so as to control the profile of the edges of the rolled product.
According to another characteristic of the invention, the crossing mechanism allows to carry out the crossing of each intermediate roll quickly, during the rolling step, since the maximum rotation of the intermediate rolls, compared with the working rolls, is about 1.5xc2x0 and since the speed of rotation is about 0.1xc2x0/sec, the correction operation, which requires to vary the angle by 0.2-0.6xc2x0, is carried out in about 2-6 secs.
According to another characteristic of the invention, the method to control the planarity of the plane rolled product provides a step of monitoring, by sensor means, the profile of the product emerging from the stand, and a step of acting on shifting means and bending means associated with at least one of the working rolls to translate it axially and respectively bend it, and on crossing means associated with the intermediate roll to arrange it with its longitudinal axis inclined, that is, rotated with respect to the longitudinal axes of the working rolls and the back-up rolls.
With reference to FIG. 16, in which xe2x80x9c0xe2x80x9d indicates the work point which represents the profile of the strip with all the actuators in the inactive position, that is to say, a xe2x80x9cnatural standxe2x80x9d position, and xe2x80x9cFxe2x80x9d indicates the point which represents the strip profile desired, it should be remembered that to obtain the desired profile it is necessary, according to the invention, to make the following two operations:
Set WR shifting to suitably correct the edge profile (xe2x80x9cedge-drop compensationxe2x80x9d); from point xe2x80x9c0xe2x80x9d we pass to an intermediate point xe2x80x9c1xe2x80x9d;
Act on WR bending and IR crossing so as to pass from point xe2x80x9c1xe2x80x9d to point xe2x80x9cFxe2x80x9d, according to the diagram shown in FIG. 16.